The present invention is related to semiconductor devices and, in particular, to solar cells.
A conventional semiconductor solar cell is made of a p-n junction. One side of the junction is usually very shallow to allow photons to be penetrated to the junction area. The photons break up the semiconductor atoms into holes and electrons, which cross the junction and flow as load current. For the current to flow into the load, ohmic contacts must be made to both the p-region and the n-region.
In the semiconductor art, ohmic contacts are usually made by allowing a metal to the semiconductor at or slightly below the eutectic temperature. This alloying process is also known as sintering. In the sintering process, the metal penetrates into the semiconductor to form an alloy. If the one side of the junction is very thin as is typically the case for a solar cell, contacting metal for the thin layer may penetrate through the layer and cause a short circuit between the metal and the other underlying side of the junction. If no high temperature sintering is used, the contact may not be ohmic, and the series resistance can be very high.
With the advent of ion-implantation, the problem is particularly severe. For a diffused junction, the junction depth is typically in the 1 micrometer range. For an implanted junction, the range is typically a tenth of a micrometer or less. An ion-implanted layer offers a number of advantages such as improved blue spectrum response, better lifetime due to low temperature processing, low energy consumption, better control of doping, etc. To realize these advantages, one must be able to make an ohmic contact to the thin ion-implanted layer.